The process of including a new module into the Python standard library is
hindered by the API lock-in and promise of backward compatibility implied by
a module being formally part of Python. This PEP proposes a transitional
state for modules - inclusion in a special __preview__ package for the
duration of a minor release (roughly 18 months) prior to full acceptance into
the standard library. On one hand, this state provides the module with the
benefits of being formally part of the Python distribution. On the other hand,
the core development team explicitly states that no promises are made with
regards to the module's eventual full inclusion into the standard library,
or to the stability of its API, which may change for the next release.

Based on his experience with a similar "labs" namespace in Google App Engine,
Guido has rejected this PEP [3] in favour of the simpler alternative of
explicitly marking provisional modules as such in their documentation.

If a module is otherwise considered suitable for standard library inclusion,
but some concerns remain regarding maintainability or certain API details,
then the module can be accepted on a provisional basis. While it is considered
an unlikely outcome, such modules may be removed from the standard library
without a deprecation period if the lingering concerns prove well-founded.

As part of the same announcement, Guido explicitly accepted Matthew
Barnett's 'regex' module [4] as a provisional addition to the standard
library for Python 3.3 (using the 'regex' name, rather than as a drop-in
replacement for the existing 're' module).

Whenever the Python core development team decides that a new module should be
included into the standard library, but isn't entirely sure about whether the
module's API is optimal, the module can be placed in a special package named
__preview__ for a single minor release.

In the next minor release, the module may either be "graduated" into the
standard library (and occupy its natural place within its namespace, leaving the
__preview__ package), or be rejected and removed entirely from the Python
source tree. If the module ends up graduating into the standard library after
spending a minor release in __preview__, its API may be changed according
to accumulated feedback. The core development team explicitly makes no
guarantees about API stability and backward compatibility of modules in
__preview__.

Entry into the __preview__ package marks the start of a transition of the
module into the standard library. It means that the core development team
assumes responsibility of the module, similarly to any other module in the
standard library.

We expect most modules proposed for addition into the Python standard library
to go through a minor release in __preview__. There may, however, be some
exceptions, such as modules that use a pre-defined API (for example lzma,
which generally follows the API of the existing bz2 module), or modules
with an API that has wide acceptance in the Python development community.

In any case, modules that are proposed to be added to the standard library,
whether via __preview__ or directly, must fulfill the acceptance conditions
set by PEP 2.

It is important to stress that the aim of this proposal is not to make the
process of adding new modules to the standard library more difficult. On the
contrary, it tries to provide a means to add more useful libraries. Modules
which are obvious candidates for entry can be added as before. Modules which
due to uncertainties about the API could be stalled for a long time now have
a means to still be distributed with Python, via an incubation period in the
__preview__ package.

In principle, most modules in the __preview__ package should eventually
graduate to the stable standard library. Some reasons for not graduating are:

The module may prove to be unstable or fragile, without sufficient developer
support to maintain it.

A much better alternative module may be found during the preview release

Essentially, the decision will be made by the core developers on a per-case
basis. The point to emphasize here is that a module's appearance in the
__preview__ package in some release does not guarantee it will continue
being part of Python in the next release.

Suppose the example module is a candidate for inclusion in the standard
library, but some Python developers aren't convinced that it presents the best
API for the problem it intends to solve. The module can then be added to the
__preview__ package in release 3.X, importable via:

from __preview__ import example

Assuming the module is then promoted to the standard library proper in
release 3.X+1, it will be moved to a permanent location in the library:

Currently, the core developers are really reluctant to add new interfaces to
the standard library. This is because as soon as they're published in a
release, API design mistakes get locked in due to backward compatibility
concerns.

By gating all major API additions through some kind of a preview mechanism
for a full release, we get one full release cycle of community feedback
before we lock in the APIs with our standard backward compatibility guarantee.

We can also start integrating preview modules with the rest of the standard
library early, so long as we make it clear to packagers that the preview
modules should not be considered optional. The only difference between preview
APIs and the rest of the standard library is that preview APIs are explicitly
exempted from the usual backward compatibility guarantees.

Essentially, the __preview__ package is intended to lower the risk of
locking in minor API design mistakes for extended periods of time. Currently,
this concern can block new additions, even when the core development team
consensus is that a particular addition is a good idea in principle.

For future end users, the broadest benefit lies in a better "out-of-the-box"
experience - rather than being told "oh, the standard library tools for task X
are horrible, download this 3rd party library instead", those superior tools
are more likely to be just be an import away.

For environments where developers are required to conduct due diligence on
their upstream dependencies (severely harming the cost-effectiveness of, or
even ruling out entirely, much of the material on PyPI), the key benefit lies
in ensuring that anything in the __preview__ package is clearly under
python-dev's aegis from at least the following perspectives:

Licensing: Redistributed by the PSF under a Contributor Licensing Agreement.

Documentation: The documentation of the module is published and organized via
the standard Python documentation tools (i.e. ReST source, output generated
with Sphinx and published on http://docs.python.org).

PEP 407 proposes a change to the core Python release cycle to permit interim
releases every 6 months (perhaps limited to standard library updates). If
such a change to the release cycle is made, the following policy for the
__preview__ namespace is suggested:

For long term support releases, the __preview__ namespace would always
be empty.

New modules would be accepted into the __preview__ namespace only in
interim releases that immediately follow a long term support release.

All modules added will either be migrated to their final location in the
standard library or dropped entirely prior to the next long term support
release.

Python already has a "forward-looking" namespace in the form of the
__future__ module, so it's reasonable to ask why that can't be re-used for
this new purpose.

There are two reasons why doing so not appropriate:

1. The __future__ module is actually linked to a separate compiler
directives feature that can actually change the way the Python interpreter
compiles a module. We don't want that for the preview package - we just want
an ordinary Python package.

2. The __future__ module comes with an express promise that names will be
maintained in perpetuity, long after the associated features have become the
compiler's default behaviour. Again, this is precisely the opposite of what is
intended for the preview package - it is almost certain that all names added to
the preview will be removed at some point, most likely due to their being moved
to a permanent home in the standard library, but also potentially due to their
being reverted to third party package status (if community feedback suggests the
proposed addition is irredeemably broken).

One proposed alternative [1] was to add explicit versioning to the
__preview__ package, i.e. __preview34__. We think that it's better to
simply define that a module being in __preview__ in Python 3.X will either
graduate to the normal standard library namespace in Python 3.X+1 or will
disappear from the Python source tree altogether. Versioning the _preview__
package complicates the process and does not align well with the main intent of
this proposal.

It was proposed [1] to use a package name like preview or exp, instead
of __preview__. This was rejected in the discussion due to the special
meaning a "dunder" package name (that is, a name with leading and
trailing double-underscores) conveys in Python. Besides, a non-dunder name
would suggest normal standard library API stability guarantees, which is not
the intention of the __preview__ package.

A pickled class instance based on a module in __preview__ in release 3.X
won't be unpickle-able in release 3.X+1, where the module won't be in
__preview__. Special code may be added to make this work, but this goes
against the intent of this proposal, since it implies backward compatibility.
Therefore, this PEP does not propose to preserve pickle compatibility.

Dj Gilcrease initially proposed the idea of having a __preview__ package
in Python [2]. Although his original proposal uses the name
__experimental__, we feel that __preview__ conveys the meaning of this
package in a better way.